Iran may suspect special lizards of spying, but the use of animals for intelligence purposes dates back well over 100 years and involves not just reptiles but cats, dogs, birds and even sea life.

Carrier pigeons were used in ancient times for relaying messages. But interest in the use of animals has changed with the development of microelectronics and miniaturization that allowed small listening devices to be put on birds and even small mammals.

More recently, technology has been catching up with dog-like robots for defense use as well as hummingbird-size drones tested by the Pentagon. The Air Force also has released video of “bugbots” or “birdbots” that could be used for surveillance and military applications, including potential swarm attacks.

Iran has a long history of suspecting animals for spying, particularly accusing the West of trying to gather information about its nuclear activities.

Back in 2008, two “spy pigeons” were suspected of being used to gather intelligence about Iran’s uranium enrichment plant in Natanz, reported Iran’s reformist paper Etemad Melli. It said one of the birds was captured not far from the heavily bunkered underground facility and had metal rings, strings and other suspicious features attached.

Iran’s media also reported the case of 14 “spy” squirrels that were busted in 2007. The account at the time by the daily Resalat claimed the rodents were released along its border by Western intelligence and fitted with espionage equipment, including navigation tracking, bugging devices and a camera.

As for lizards spying, the stories about the reptiles surfaced Tuesday when Hassan Firuzabadi, a senior military advisor to Iran’s supreme leader Ayatollah Ali Khamenei, told the state-run Iranian Labour News Agency about how lizards and perhaps salamanders were used by Western countries to “find out where we had uranium mines and where we were involved in atomic activities.”

According to Firuzabadi, “lizard-like animal skins attract nuclear waves.” He claimed Iranian authorities stumbled on suspicious cases of outsiders with reptiles in their possession and concluded it was part of a pattern of espionage conducted by environmentalists.

“Probably the reason the Iranians are paranoid and jumpy is because people have used fake rocks outside Iranian nuclear facilities to monitor what they’re up to,” said James Lewis, a senior vice president at the Center for Strategic and International Studies, a Washington-based international security think tank.

Lewis, a former U.S. diplomat with experience in high technology and intelligence, said the rocks reportedly would self-destruct when they were picked up. The rocks were found by Iran’s Revolutionary Guards on patrol near the country’s underground nuclear enrichment facility in Fordow and reported first in 2012 by U.K.’s Sunday Times newspaper.

Similarly, Iran-backed militant groups also have accused Israel of using animals for espionage.

In 2015, Gaza Strip’s Hamas security officials reportedly captured a dolphin equipped with “video cameras” off the coast, according to the Palestinian paper al-Quds. The Iran-backed group claimed the dolphin was sent by Israel and also fitted with a weapon that could fire arrows at humans.

There also was a 2016 case of a “spy vulture” captured in the southern Lebanon town of Bint Jbeil. Local media in Lebanon called it a “spy” bird because it reportedly carried transmitter equipment, but Israel claimed it was from a nature reserve and asked for it back. Parts of southern Lebanon are controlled by Hezbollah, an Iranian-backed terrorist group.

There have also been claims over the years from Saudi Arabia, Turkey and Sudan of the Israelis using eagles, vultures or other birds for espionage. An Egyptian official in 2010 claimed sharks controlled by Israel’s Mossad were responsible for attacks on tourists in the Red Sea to hurt the local tourism economy.

The U.S. Defense Advanced Research Projects Agency, or DARPA, the R&D arm of the Pentagon, has tested controlling sharks, and the U.S. Navy does training with dolphins and sea lions. There’s also been research over the decades with beluga whales.

The use of the dolphins by the U.S. military focuses primarily around locating underwater mines and helping with rescues at sea. The dolphins, which are trained at a base in San Diego, were used by the U.S. military during the first and second Gulf wars to help clear mines.

The use of marine mammals for military purposes dates back to the Cold War era and involved not just the U.S. but the Soviet Union, starting in the 1960s. By the 1980s, the U.S. Navy reportedly had as many as 100 dolphins trained for military use along with an unknown number of whales and California sea lions.

Other U.S. agencies during the Cold War were actively involved in animal research for espionage or wartime use. For example, the CIA’s 1960s so-called Acoustic Kitty project involved using trained cats to spy on the Kremlin and Soviet embassies, including the compound in Washington, D.C.

The CIA’s cat program involved planting a small transmitter above the cat’s skull and placing a microphone in the animal’s ear canal. The program reportedly didn’t work so well from a technical standpoint and due to animal behavior, so it was abandoned but still cost millions of dollars.

“The problem with training animals is they don’t always do what you ask them to do,” said Lewis.

The Germans also reportedly used cats as well as dogs for spying and crossing enemy lines during World War I. Documents released by the British from an intelligence briefing dating to 1915 stated: “Two cats and a dog are under suspicion as they have been in the habit of crossing our trenches at night: steps are being taken to trap them if possible.”

Technology in the past decade, though, has allowed the military to develop dog-like robots for possible defense use as well as hummingbird-size drones. And there’s even research and development of mosquito-like unmanned aerial vehicles.

In 2015, DARPA and Boston Dynamics, a U.S. defense contractor, showed off a 160-pound “Spot” robot at a Marine Corps base in Virginia. The dog-like robot could be used to scout or to hold equipment on the battlefield.

“During a military operation in urban terrain drill, Spot went into the building before the Marines simulating [peeking] around corners and looking for enemies and possible threats,” the Marine Corps said in a press release describing the robot testing.

Another DARPA-funded project involved creating a camera-equipped robotic hummingbird with California-based AeroVironment, a maker of small drones already used in combat. The “Nano Hummingbird” has a 6.5-inch body and can flap its wings like the real bird but doesn’t stay in flight for very long.

Drones are have been created that resemble insects, such as butterflies or mosquitoes, that could be used for espionage or even possible weapons, including for swarm-like attacks. Israel, the U.S., Russia and China have all reportedly done research on bug-like drones.

And U.S. research is currently underway that could ultimately produce what’s known as nanobots for military and espionage missions.

For one, DARPA has the Fast Lightweight Autonomy program to allow autonomous drones to enter a building and avoid hitting walls or objects. DARPA announced a breakthrough in 2016 after tests in a hangar in Massachusetts.

Previously, the Army Research Laboratory announced it created an advanced drone the size of a fly complete with a set of “tiny robotic legs” — a major achievement since it presumably might be capable of entering a building undetected to perform surveillance or used for more nefarious actions.

Frightening details about military nanotechnologies were outlined in a 2010 report from the Pentagon’s Defense Threat Reduction Agency, including how “transgenic insects could be developed to produce and deliver protein-based biological warfare agents, and be used offensively against targets in a foreign country.”

The also was a “cyborg insect” project a decade ago from DARPA called the Hybrid Insect Micro Electro-mechanical Systems (HI-MEMS) program to put electronic circuitry in real bugs so they could be controlled by humans.

The original plan was to obtain information using various types of sensors on the insects, including gas sensors, microphones and video. The HI-MEMS research showed promise but was officially halted in 2010.

“Even though technical feasibility of controlled flight of an insect-based platform was demonstrated, it was determined that the technology was too immature to justify continued funding,” DARPA told CNBC in statement.